Hi Alex
I would assume that your kernel does not have the following patch it can be
found at the nouveau repo [1]

commit bf03d1b293cc556df53545e318110505014d805e
Author: Ilia Mirkin <imirkin@alum.mit.edu>
Date:   Wed Jul 3 03:06:02 2013 -0400

    drm/nva3/disp: Fix HDMI audio regression

    This is the nva3 counterpart to commit beba44b17 (drm/nv84/disp: Fix
    HDMI audio regression). The regression happened as a result of
    refactoring in commit 8e9e3d2de (drm/nv84/disp: move hdmi control into
    core).

    Reported-and-tested-by: Max Baldwin <archerseven@gmail.com>
    Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
    Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
    Cc: stable@vger.kernel.org


Note that HDMI audio is not fully reverse-engineered, thus you may be one of
the "lucky" people where it does a bit of extra in order to work :)

[1] http://cgit.freedesktop.org/nouveau/linux-2.6/